Soil and cultivation substrate are mixed component of mineral and organic matter, which exist solid, gas, and liquid states and belong to heterogeneous environment. Liquids and gases are mainly existed in the gap between solid particles. The complex composition and heterogeneous mixture environments lead totally different physical and chemical properties of the soil, which make it difficult to measure the physical and chemical properties of the soil and cultivation substrate in-situ.
It is essential to detect the concentration of hydrogen ions during pH measurement of soils and cultivation substrates. The hydrogen ions are mainly produced by acid ionization in water. Hydrogen ions are “naked” protons with tiny radius, which turn into hydronium ions (H3O+) through combined with other water molecules easily. Hydrogen ions exist in the form of hydronium ions usually. Therefore, the hydrogen ions in the soil are closely related to the water in the soil. To realize the in-situ measurement of the pH of heterogeneous systems such as soil substrates, it is necessary to find a way to quickly absorb the water in the soil. Then hydrogen ions are adsorbed, and the pH of soil culture substrate can be measured in-situ quickly.
The research of existing pH sensor mainly focused on electrode materials, electrode modification methods and composite electrode preparation. Li-Min Kuo prepared a novel pH microsensor in his paper “A precise pH microsensor using RF-sputtering IrO2 and Ta2O5 films on Pt-electrode” which used Ta2O5 to modify the surface of IrO2 electrode by magnetron sputtering technology. In the paper of “A potentiometric disposable sensor strip for measuring pH in saliva”, Claudio Zuliani used screen printing technology to print a solid-state reference electrode and ion sol reference electrode on a printed electrode substrate, then formed an all-solid-state complex pH electrode for rapid detection of pH in oral saliva. These studies extended the use of pH electrode to a certain extent and effectively improved the response performance of pH electrodes, such as sensitivity, stability and anti-interference. However, there is no effective solution for pH in-situ measurement in heterogeneous systems and how to allow the electrodes to adsorb water in the soil and cultivate substrates rapidly, and then absorb the hydrogen ions.